1. Field of the Invention
The present invention relates to an inkjet printer and a bubble removing method in an inkjet printer, and more particularly, to an inkjet printer and a bubble removing method in an inkjet printer having a structure for removing bubbles in an ink-flow passage.
2. Description of Related Art
An inkjet printer that ejects ink on a recording medium, such as a paper and a plastic sheet, to record a given image thereon, has been proposed and put into practice. Such an inkjet printer has a recording head for ejecting ink supplied from an ink tank through an ink-flow passage. In a serial-type inkjet printer, for example, while a recording head mounted on a carriage moves in a predetermined direction, ink is ejected from the recording head toward a recording medium to record a given image on the recording medium. In a line-type inkjet printer, ink is ejected from a line-type recording head onto a recording medium transported in a predetermined transporting direction to record a given image on the recording medium.
In the aforementioned inkjet printer, as shown in FIG. 8, a filter 92 for removing contamination in ink is often disposed in an ink-flow passage 90 which is provided between an ink tank (not shown) and a pressure chamber 84 in a recording head 80. In this inkjet printer, a bubble K2 gets into the ink-flow passage 90 sometimes. The bubble K2 got into the ink-flow passage 90 provided between the ink tank and the filter 92 is easily trapped by the filter 92, to stay at the upstream side of the filter 92. The bubble K2 staying at the upstream side of the filter 92 clogs up the filter to form flow-passage resistance which causes ejection failures such as non-ejection, reduction of droplet volume or the like.
In detail, when a bubble stays at the upstream of a filter, the bubble forms meniscuses on filter meshes, which makes the area for the ink to pass through the filter smaller, to lower a pressure due to the ink flowing into the recording head from the ink-flow passage, resulting in the rise of backpressure to the recording head. When the backpressure to the recording head rises over a predetermined value, a meniscus formed at an ejection opening of a nozzle in the recording head breaks and a bubble gets into the pressure chamber of the recording head from the ejection opening. The bubble causes reduction of the pressure in the pressure chamber necessary for ejecting ink and may thereby cause ejection failures in the recording head, e.g., non-ejection, decrease of droplet volume or the like.
Conventionally, it has been said that a bubble retained at the upstream of the filter passes through the filter and stay in the pressure chamber of the head, and the bubble reduces the pressure in the pressure chamber necessary for ejecting ink. However, since the bubble passed through the filter is very small, it is thought that the bubble is dissolved in the ink before it reaches the pressure chamber. Therefore, in many cases, it is thought that reduction of the pressure in the pressure chamber which is necessary for ejecting ink, is actually caused by the bubble got in the pressure chamber through the ejection opening because of meniscus break due to the rise of backpressure. It is very important to solve this problem.
In order to remove the bubbles K2 retained at the upstream of the filter 92, a method in which a suction device sucks the bubble K2 together with ink from the ejection opening (orifice) 83 of a nozzle 82 to suck out the bubble K2 through the orifice, has been employed. In the method, a high pressure is needed for the bubble K2 to pass through the filter 92. In order to generate such a high pressure, a method for removing the bubble K2, including the steps of, providing a valve 95 at the upstream of the filter 92 in the ink-flow passage 90, performing suction by the suction device through the orifice 83 of the nozzle 82 with the valve 95 closed, and thereafter opening the valve 95 to accelerate ink-flow speed, is proposed (see, for example, JP-2000-289225A).
However, even if suction is performed by the suction device through the nozzle orifice with the valve closed and thereafter the valve is opened, as described above, the bubbles actually cannot be removed because the ink-flow speed does not increase, unless a differential pressure between the upstream and the downstream of the filter reaches a value to cause meniscus break so that the ink at the upstream of the filter flows into the downstream side through the filter.
In case that the ink-flow passage at the downstream of the filter, that is, the ink-flow passage between the filter and the recording head is filled with ink without a bubble, the volume of liquid ink hardly changes to a pressure even by sucking with the valve closed.
At the upstream of the filter, on the contrary, the bubble expands due to negative pressure when sucked with the valve closed. However, because the volume of liquid ink hardly changes at the filter downstream, the expanded bubble just shrinks to return to the original volume when the valve is opened, and therefore the meniscus does not break and the bubble does not flow into the downstream side through the filter.
Accordingly, the bubble remains in the ink-flow passage at the filter upstream for a long time. As a result, it sometimes causes lasting malfunction such as ejection failures in the head.
In a conventional inkjet printer, a recording head has not so many nozzles in most cases and a flow passage has a small flow resistance due to a small cross-section in most cases, therefore removal of bubble can be achieved relatively easily by suction without particular consciousness about the above-described problem. However, recently, a large-scale recording head with many nozzles has been manufactured for speed up of image recording and use of a line-type head. The larger recording head causes the flow-passage cross-section to be larger, and therefore a filter area is necessitated to be relatively larger to reduce the flow-passage resistance. Resultantly, bubbles tend to remain at the filter upstream. Therefore, when removing bubbles using a valve in a large-sized recording head, it is necessary not to simply raise the flowing speed but to consciously generate large differential pressure between the upstream and the downstream of the filter so as to break the meniscus.